Blastocyst Embryo Transfer

Blastocyst transfer achieved the first IVF human pregnancy. Blastocyst transfer is claimed to be more physiological than pronucleate or cleaved-embryo transfer is as it mimics nature more closely. As the embryo advances in the development, after 5-6 days it becomes a blastocyst(see picture). This has an outer thin layer of cells, which will later form the placenta, and an inner cell mass, which will develop into the fetus. A blastocyst has about 120 cells. A blastocyst gives a better idea of the competence of an embryo and has a higher chance of implantation than a cleaved embryo. In conventional culture medium, about 20% of embryos will develop into blastocysts. Recently, the use of sequential culture medium (the embryos are cultured in different media according to their stage of growth) has enabled a larger number of embryos to develop into blastocysts. However, up to 40% of patients will not grow blastocysts and will not have blastocyst embryo transfer. The rationale behind a blastocyst transfer is that an embryo, which has failed to reach the blastocyst stage, would be unlikely to have resulted in a pregnancy. However, if it reaches the blastocyst stage it has about 50% chance of implanting. So the improved implantation rates following blastocyst transfer is due to selection of the best embryos.
Why then do 50% of the blastocysts fail to implant? A defective blastocyst (e.g. chromosomal abnormalities) is a possible cause; a non-receptive endometrium is another cause. Blastocyst embryo transfer into the uterine cavity is performed about 5-6 days after egg collection. Transfer of one or two blastocysts is recommended to avoid high-order multiple pregnancies. Supernumerary blastocysts can be frozen for future use.
Blastocyst transfer is recommended for patients who had repeatedly failed to achieve a pregnancy following the transfer of good quality cleaved embryos (If the embryo arrests and did not develop to blastocyst, this may indicate a potential egg problem). Patients who wish to achieve a pregnancy without the risk of multiple pregnancies will benefit from a single blastocyst transfer. Patients who do not wish to have their spare embryos frozen for whatever reasons may be advised to have blastocyst transfer. About 10% of the embryos that fail to develop to blastocyst in vitro may have done so if replaced inside the womb on day 2 or 3. Up to 40% of patients will not have blastocysts available for transfer. Freezing spare blastocysts was not as good as freezing cleaved embryos in the past. But, with the advent of Vitrification, high pregnancy rates have been reported. We, at Rotunda have a highly successful vitrification program for cleavage stage embryos as well as blastocysts.


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